GathenKnopfmacherLucaLuchtSchparlinsky. 291KB Jun 04 2011 12:08:41 AM
Journal de Th´eorie des Nombres
de Bordeaux 16 (2004), 107–123
Average order in cyclic groups
par Joachim von zur GATHEN, Arnold KNOPFMACHER,
Florian LUCA, Lutz G. LUCHT et Igor E. SHPARLINSKI
´sume
´. Pour chaque entier naturel n, nous d´eterminons l’ordre
Re
moyen α(n) des ´el´ements du groupe cyclique d’ordre n. Nous montrons que plus de la moiti´e de la contribution `a α(n) provient des
ϕ(n) ´el´ements primitifs d’ordre n. Il est par cons´equent int´eressant
d’´etudier ´egalement la fonction β(n) = α(n)/ϕ(n). Nous d´eterminons le comportement moyen de α, β, 1/β et consid´erons aussi
ces fonctions dans le cas du groupe multiplicatif d’un corps fini.
Abstract. For each natural number n we determine the average
order α(n) of the elements in a cyclic group of order n. We show
that more than half of the contribution to α(n) comes from the
ϕ(n) primitive elements of order n. It is therefore of interest to
study also the function β(n) = α(n)/ϕ(n). We determine the
mean behavior of α, β, 1/β, and also consider these functions in
the multiplicative groups of finite fields.
Section 1. Introduction
For a positive integer n, we determine the average order α(n) of the
elements in the additive cyclic group Zn of order n. The major contribution
to α(n) is from the ϕ(n) primitive elements in Zn , each of order n. We show
that, in fact, the other elements never contribute more than the primitive
ones do.
More precisely, we consider the relative version β(n) = α(n)/ϕ(n). With
A=
315 ζ(3)
ζ(2)ζ(3)
=
≈ 1.94359 64368
ζ(6)
2π 4
we have for n ≥ 2:
1 = lim inf β(n) < β(n) < lim sup β(n) = A.
n−→∞
n−→∞
We also determine the mean behavior of α, β, and 1/β, and discuss the
average order of elements in the multiplicative groups of finite fields. The
lower bounds for β are different for even and for odd characteristic.
Manuscrit re¸cu le 19 avril 2002.
108
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
The original motivation for this research was the usage of groups in
cryptography. Here one looks for cyclic groups of large order (preferably a
prime number). If we take a finite field and pick a random element from
it, how large can we expect its order to be? Intuition says that one should
avoid fields whose multiplicative group order is largely made up from small
prime factors. The results of this paper put this intuition on a firm basis.
Section 2. The average order
For a ∈ Zn , we denote by ord(a) its order in the additive group Zn .
Then ord(a) divides n, and for each divisor d of n, there are exactly ϕ(d)
elements in Zn of order d. Thus the average order in Zn is
1X
1 X
ord(a) =
dϕ(d).
α(n) =
n
n
a∈Zn
d|n
The main contribution is the term with d = n, and we normalize by it:
α(n)
.
β(n) =
ϕ(n)
Since 1/n and ϕ(n) are multiplicative functions of n, so is their Dirichlet
convolution α(n) (see Apostol 1976, Theorem 2.14), and also β(n). We first
determine their values in the case of a prime power.
Lemma 2.1. Let p be a prime and k ≥ 1 an integer. Then
pk+1
1
1
1
k
k
α(p ) =
+
1 + 2k−1 .
, β(p ) = 1 + 2
p + 1 pk (p + 1)
p −1
p
In particular β(1) = 1 < β(pk+1 ) < β(pk ) ≤ β(p) = 1 + 1/(p2 − p).
Proof.
We have
X
1 X i
1
k
2i−1
i
α(p ) = k
(p − 1) · p
p ϕ(p ) = k 1 +
p
p
1≤i≤k
0≤i≤k
=
p2k+1
pk (p
+1
,
+ 1)
p2k+1 + 1
1
1
β(p ) = k
1 + 2k−1 .
= 1+ 2
p −1
p
p (p + 1)(p − 1)pk−1
k
Theorem 2.2. For an integer n ≥ 2, we have the following inequalities.
Y
Y
1
1
< β(n) ≤
1+
< A.
(i) 1 ≤
1+ 2
p −1
p(p − 1)
p|n
p|n
(ii) 1 = lim inf β(n) < β(n) < lim sup β(n) = A.
n−→∞
n−→∞
Average order in cyclic groups
Proof.
We have
Y
p prime
=
Y
p prime
1
1+
p(p − 1)
=
109
Y 1 − p−1 + p−2
1 − p−1
p prime
p−6
ζ(2)ζ(3)
1−
=
= A.
−2
−3
(1 − p )(1 − p )
ζ(6)
Claim (i) now follows from the multiplicativity of β and the lemma. For
(ii), we clearly have 1 < β(n) < A for all n ≥ 2. When n ranges through
1
the primes, then β(n) = 1 + n(n−1)
tends to 1, and when nk is the product
of the first k primes, then limk→∞ β(nk ) = A.
1.8
1.6
1.4
1.2
1
0
200
400
600
800
1000
Figure 2.1. Relative average order β(n) for n ≤ 1000.
Figure 2.1 shows the behavior of β(n) for n ≤ 1000. The visible bands
at 1 = β(1), 1.5 = β(2), 1.17 ≈ β(3), for example, are created by numbers
of the form n = kp with small k and p either prime or having only large
prime factors, namely k = 1, 2, 3 for the bands mentioned.
We have seen that α(n) is firmly wedged between ϕ(n) and A · ϕ(n).
Since lim inf n→∞ ϕ(n)/n = 0, we also have
lim inf α(n)/n = 0.
n→∞
Theorem 4.4 below shows that this lower limit is even obtained on subsequences corresponding to the multiplicative groups of finite fields.
110
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
Our upper limit A occurs in several other contexts. Kendall & Rankin
(1947), Section 3, consider the number of divisors of n that are divisible
by the squarefree part of n, and show that its asymptotic mean value is
A. Knopfmacher (1973) gives a more precise description of the mean value,
and Knopfmacher (1972), Theorem 3.1 (vi), presents a generalization. The
moments of this function are studied in Knopfmacher & Ridley (1974),
Theorem 4.4. LeVeque (1977), Problem 6.5, determines A as the sum given
in (3.7) below, and shows that the asymptotic mean of 1/ϕ is Ax−1 log x.
The constant A also appears in Bateman (1972), Montgomery (1970), and
Riesel & Vaughan (1983).
Throughout the paper, log x is the natural logarithm of x.
Section 3. The mean average order
In this section, we determine the mean of the averaging functions α and
β, and of γ = 1/β. A pleasant feature, due to double averaging, is that the
error terms become rather small. We denote the average of an arithmetic
function g by g¯—not to be confused with complex conjugation:
1 X
g(n)
g¯(x) =
x
1≤n≤x
for x ≥ 1. There is a well-developed theory with many general results
about the existence of means of arithmetic functions, see Elliott (1985);
Indlekofer (1980, 1981); Postnikov (1988). However, those general results
do not imply the specific statements of this work.
The average α
¯ is connected to the constant
3ζ(3)
ζ(3)
=
≈ 0.36538 14847.
Cα =
2ζ(2)
π2
Theorem 3.1. The mean α
¯ of α satisfies
α
¯ (x) = Cα x + O((log x)2/3 (loglog x)4/3 ) for x ≥ 3.
We have
1 X 1X
1 X 1 X
1 X
α(n) =
dϕ(d) =
ϕ(d).
α
¯ (x) =
x
x
n
x
k
Proof.
1≤n≤x
1≤n≤x
d|n
1≤k≤x
d≤x/k
Walfisz (1963), Chapter IV, proves via exponential sum estimates that
x
¯
−
ϕ(x)
≤ c (log x)2/3 (loglog x)4/3 for x ≥ 3,
2ζ(2)
with some constant c. Now from
Z ∞
X 1
dt
1
≤
=
3
3
k
2(x − 1)2
x−1 t
xx
1≤d≤x
112
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
By inserting the estimates
Z ∞
X 1
dt
1
≤
=
,
2
2
d
⌊x⌋
⌊x⌋ t
d>x
Z x
X 1
dt
≤1+
= 1 + log x,
d
1 t
1≤d≤x
we see that for x ≥ 2
1
|R(x)| ≤ 1 + log x +
2
1
x
+
⌊x⌋ x
< 2 + log x.
This shows (i), and (ii) follows by inserting (i) into the proof of Theorem 3.1.
For two arithmetic functions f, g : N −→ C, f ∗ g is their Dirichlet convolution, with
X
(f ∗ g)(n) =
f (d)g(n/d)
d|n
for all n. Furthermore, we denote by 1 the constant function on N with
value 1, and µ is the M¨obius function.
Lemma 3.3. Let f and g be arithmetic functions with f = 1 ∗ g, and
consider the Dirichlet series
X g(n)
ge(s) =
.
ns
n≥1
(i) If ge(s) is absolutely convergent for ℜs ≥ 0, then the mean of f is
1
¯
,
f (x) = ge(1) + O
x
and more precisely
|f¯(x) − ge(1)| ≤
1X
|g(n)|
x
n≥1
for x ≥ 1.
(ii) If f is multiplicative and ge(s) converges absolutely for some s with
ℜs ≥ 0, then ge(s) can be written as the Euler product
Y
g(p) g(p2 )
ge(s) =
1 + s + 2s + · · · .
p
p
p prime
The absolute convergence of ge(s) is equivalent to
X |f (pk ) − f (pk−1 )|
< ∞.
pks
p prime
k≥1
Average order in cyclic groups
Proof.
113
For x ≥ 1, we have
X
X
x
g(d)
=
f
(n)
−
xe
g
(1)
−
xe
g
(1)
d
d≤x
n≤x
X g(d)
X
≤ x
− xe
g (1) +
|g(d)|
d
d≤x
≤x
d≤x
X |g(d)|
d>x
d
+
X
|g(d)| ≤
d≤x
X
|g(d)|,
1≤d
which implies (i).
If f is multiplicative, then so is g = µ ∗ f , and g(pk ) = f (pk ) − f (pk−1 )
for all primes p and k ∈ N . Now the Euler product representation of
ge(s) follows from the unique factorization
If ge(s) is absolutely
P theorem.
convergent, then so is the partial series
g(pk ) p−ks taken over all prime
powers pk . Conversely, absolute convergence of the latter series implies that
for any x ≥ 1
X |g(pk )|
Y
X |g(n)|
1+
≤
nℜs
pkℜs
p≤x
p prime
n≤x
≤
Y
p≤x
p prime
k≥1
exp
X
k≥1
|g(pk )|
pkℜs
≤ exp
X
p prime
k≥1
|g(pk )|
pkℜs
Thus ge(s) converges absolutely, which finishes the proof of (ii).
< ∞.
The mean of β is connected to the constant
105 ζ(3)
ζ(3)ζ(4)
=
≈ 1.29573 09579.
Cβ =
ζ(8)
π4
Theorem 3.4. The average value β¯ of β equals Cβ + O(x−1 ), and more
precisely
Y
p+2
−1
¯
|β(x) − Cβ | < x
1+ 3
p −p
p prime
for x ≥ 1.
Proof.
We use Lemma 3.3 with f = β and g = µ ∗ β. Thus
1
for k = 1,
1)
g(pk ) = β(pk ) − β(pk−1 ) = p(p −
1
−
for k ≥ 2,
p2k−1
114
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
for a prime p and an integer k ≥ 1 . Due to
X 1
X
p+2
1
= 3
+
,
(3.5)
|g(pk )| =
p (p − 1)
p −p
p2k−1
k≥2
k≥1
P
the series
|g(pk )| p−kℜs taken over all prime powers pk converges for
ℜs ≥ 0, and Lemma 3.3 (ii) implies the absolute convergence of the Dirichlet
series ge(s). In particular, we obtain
Y
X 1
Y
p+1
1
=
1
+
−
ge(1) =
1+ 2
p (p − 1)
p (p3 − 1)
p3k−1
p prime
=
Y
p prime
p prime
k≥2
Y
p−4
1+
=
1 − p−3
p−8
1−
ζ(3) ζ(4)
=
= Cβ .
(1 − p−3 )(1 − p−4 )
ζ(8)
p prime
Finally, Lemma 3.3 combined with (3.5) yields
X
Y
X
¯
|β(x)
− Cβ | < x−1
|g(d)| = x−1
1+
|g(pk )|
p prime
d≥1
= x−1
Y
1+
p prime
p+2
p3 − p
k≥1
≈ 2.26507 69892 · x−1 ,
which completes the proof.
¯
It is interesting to compare the behavior of β(x)
≈ ζ(3)ζ(4)/ζ(8) with
its naive “prediction” α(x)/
¯
ϕ(x)
¯
≈ ζ(3), see Theorem 3.1 and Lemma 3.2.
We have ζ(4)/ζ(8) ≈ 1.07792 81367.
Figure 3.1 shows the behavior of
Y
p + 2 −1
¯
1+ 3
(3.6)
(β(x)
− Cβ ) · x ·
p −p
p prime
for integer x ≤ 1000. Theorem 3.4 says that this quantity is absolutely
smaller than 1.
We can also express our constants A and Cβ as sums of Dirichlet series
via the Euler product decomposition
Y
X
f (n) =
(1 + f (p) + f (p2 ) + · · · ),
1≤n
p prime
which is valid for a multiplicative function f in the case of absolute convergence. Now
1+
X (µ(pk ))2
1
,
=
p(p − 1)
pk ϕ(pk )
0≤k
1+
X
1
(µ(pk ))2
=
p(p3 − 1)
pk ϕ(pk )σ(p2k )
0≤k
Average order in cyclic groups
115
0.05
200
400
600
800
1000
0
–0.05
–0.1
–0.15
–0.2
Figure 3.1. The average of β normalized as in (3.6).
imply that
(3.7)
A=
X (µ(n))2
1≤n
nϕ(n)
,
Cβ =
X
1≤n
(µ(n))2
.
nϕ(n)σ(n2 )
Both series seem to converge much slower than the product representations.
The mean of the function γ = 1/β = ϕ/α is connected to the constant
Y
1 X
1
1
1+ 1− 2
Cγ =
1−
≈ 0.80146 96934.
p
p
pk + p−k−1
p prime
1≤k
Theorem 3.8. The mean γ¯ of γ satisfies γ¯ (x) = Cγ + O(x−1 ), and more
precisely,
|¯
γ (x) − Cγ | ≤ Dx−1
for a constant D which is explicitly given in the proof below.
Proof. Again, we use Lemma 3.3, with the multiplicative function f = γ.
For a prime p and k ≥ 1, we have
f (pk ) =
1 − p−2
1
=
,
β(pk )
1 + p−2k−1
116
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
by Lemma 2.1. For the multiplicative function g = µ ∗ f we find
1 + p−1
− 2
if k = 1,
p (1 + p−3 )
g(pk ) = f (pk ) − f (pk−1 ) =
1
if k ≥ 2.
2k−1
−2k+1
p
(1 + p
)(1 + p−2k−1 )
Thus the Dirichlet series ge(s) is absolutely convergent for ℜs ≥ 0. We have
X g(n)
Y
X g(pk )
.
ge(1) =
=
1+
n
pk
1≤n
p prime
1≤k
For a prime p, the factor in this product equals
X f (pk ) − f (pk−1 )
X g(pk )
1 X
1 f (pk )
1
−
+
1+
=
1
+
=
1
−
p
p
pk
pk
pk
1≤k
1≤k
1≤k
1
1
1 X
= 1−
.
1+ 1− 2
p
p
pk + p−k−1
1≤k
Lemma 3.3 now implies that
|¯
γ (x) − Cγ | ≤
1X
|g(n)|,
x
1≤n
and the claim follows from the absolute convergence of ge(0). A numerical
evaluation of the error term gives
Y
X
X
k
|g(n)| =
1+
|g(p )| ≈ 1.96531.
D=
1≤n
p prime
1≤k
We have Cβ · Cγ ≈ 1.03848 90929.
Section 4. Finite fields
Our original motivation for this work was to study the average order
in the (cyclic) multiplicative group F×
q = Fq \{0} of a finite field Fq . We
first show that for the two families q = 2k and q a prime, α(q − 1)/(q − 1)
is on average between two positive constants, and also exhibit subfamilies
for which this quotient tends to zero. We also obtain several results for
β(q − 1).
Theorem 4.1. There are two absolute constants A2 ≥ A1 > 0 such that
for all K ≥ 1
1 X α(2k − 1)
A1 ≤
≤ A2 .
K
2k − 1
1≤k≤K
Average order in cyclic groups
Proof.
(4.2)
117
We use the asymptotic formula from Shparlinski (1990)
1 X ϕ(2k − 1)
= η + O(K −1 log K),
K
2k − 1
1≤k≤K
with η given by the absolutely convergent series
η=
X µ(d)
≈ 0.73192,
dtd
d odd
where td is the multiplicative order of 2 modulo d. The claim follows from
(4.2) and Theorem 2.2.
The proof of Theorem 4.1 implies that for any constant c < η, α(2k −1) ≥
c · (2k − 1) for infinitely many integers k. We may, of course, take A2 = 1
in Theorem 4.1; it is not clear whether Theorem 4.1 holds with a smaller
value of A2 . We also see that for any ε > 0 and sufficiently large values of
K, Theorem 4.1 holds with A1 = η − ε.
Stephens (1969) shows in his Lemma 1 that
X ϕ(p − 1)
p≤x
where
κ=
p
= κ lix + O(x/(log x)D ),
Y
X µ(d)
1
=
1−
≈ 0.37397
dϕ(d)
p(p
−
1)
p
d≥1
is Artin’s constant and D > 1 is arbitrary. The sum does not change by
much if we replace p by p − 1 in the denominator, since
X ϕ(p − 1)
p≤x
p−1
=
X ϕ(p − 1)
p≤x
p
+ O(log log x).
Using the bounds of Theorem 2.2 on β = α/ϕ, the fact that p − 1 is even
for p ≥ 3, and β(2) = 3/2, we find that
(4.3)
3κ
1 X α(p − 1)
−ǫ≤
≤ Aκ + ǫ,
2
x
p−1
p≤x
for any ǫ > 0 and sufficiently large x.
Thus there is an infinite sequence of fields of characteristic 2, and also one
of prime fields, in which the average order is close to its largest possible
value. Now we show that α(2k − 1) and α(p − 1) infinitely often take
relatively small values, just as ϕ(2k − 1) and ϕ(p − 1) do.
118
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
Theorem 4.4. For infinitely many integers k ≥ 3 and for infinitely many
primes p, we have
1
1
α(p − 1)
α(2k − 1)
=O
=O
and
.
loglog k
p−1
loglog p
2k − 1
Proof.
Let pi denote the ith prime. For an integer r ≥ 1, we put
kr = (p2 − 1) · · · (pr − 1)
and
mr = p2 · · · pr = nr /2.
Then mr divides 2kr − 1, and therefore
A · (2kr − 1)ϕ(m )
r
.
α 2kr − 1 < A · ϕ 2kr − 1 ≤
mr
Using the bound
mr
ϕ(mr ) = O
log(log mr + 1)
(see Hardy & Wright (1962), Theorem 328) and kr < mr , we obtain the
first statement.
To prove the second bound, we select qr as the smallest prime number
in the arithmetic progression 1 mod mr . Then
qr
A · (qr − 1)ϕ(mr )
=O
.
α(qr − 1) < A · ϕ(qr − 1) ≤
mr
log(log mr + 1)
From Linnik’s Theorem on the smallest prime number in an arithmetic
progression, we have log qr = O(log mr ), and the result follows.
In particular,
lim inf α(2k − 1)/(2k − 1) = lim inf α(q − 1)/(q − 1) = 0.
q prime
k→∞
Open Question. Obtain analogs of (4.2) and (4.3) for the sums
X
X
X α(p − 1)
X α(2k − 1)
k
β(p−1).
,
,
β(2
−1),
p−1
2k − 1
1≤k≤K
1≤k≤K
p≤K
p≤K
In the above we considered only α(2k − 1). Similar results also hold for
α(pk − 1) for any fixed p and growing k.
The convergence to zero of α(qr − 1)/(qr − 1) as above seems rather slow.
For the largest known “primorial prime” q = n33 237 + 1, where as before
nk is the product of the first k primes (see Caldwell & Gallot 2000), with
169 966 digits and the largest prime factor p33 237 = 392 113 of q − 1, we
have α(q − 1)/(q − 1) ≈ 0.0847. Also, β(q − 1) ≈ 1.94359 608 is close to A.
Concerning lower bounds for β, the situation is quite different between
characteristic 2 and odd characteristic.
Average order in cyclic groups
119
In a finite field F2k of characteristic 2, the group of units is cyclic with
2k − 1 elements. For a Mersenne prime Mk = 2k − 1, we have β(Mk ) =
1 + (Mk2 − Mk )−1 . If there are infinitely many of them, then lim inf β(2k −
1) = 1. For the current world record k = 69 72593 (see Chris Caldwell’s
web site http://www.utm.edu/research/primes), we have β(Mk ) ≈ 1 +
0.52 · 10−41 97919 .
For a field Fq of odd characteristic, 2 divides q − 1 = #F×
q and thus
β(q − 1) > 4/3, by Lemma 2.1. For a prime q = m2k + 1 with m odd, we
have
1
4
1 + 2k+1 · β(m).
β(q − 1) =
3
2
As an example, with the prime m = 10500 + 961 and k = 3103, q is indeed
prime (Keller 2000), and
4
β(q − 1) ≈ (1 + 10−1000 ).
3
We now prove the limits indicated by these experimental results.
Theorem 4.5. We have
(i) lim inf β(p − 1) = 4/3,
p prime
(ii) lim sup β(p − 1) = A,
p prime
(iii) lim inf β(2k − 1) = 1.
k→∞
Proof. To show that the limit in (i) is at least 4/3, we notice that if
p ≥ 3, then p − 1 = 2k m with some k ≥ 1 and some odd integer m, and
therefore
4
4
1
β(p − 1) = β(2k m) =
1 + 2k+1 β(m) > .
3
3
2
For the equality in (i), we use a theorem of Chen (see Chen (1973), or
Lemma 1.2 in Ford (1999), or Chapter 11 of Halberstam & Richert (1974))
which says that for each even natural number n there exists x0 such that
for every x ≥ x0 there exists a prime number p ∈ (x/2, x] with p ≡ 1 mod n
such that (p−1)/n has at most two prime factors, and each of them exceeds
x1/10 .
We now choose a positive integer k and apply Chen’s Theorem with
n = 2k to conclude that there exist infinitely many prime numbers p such
that p − 1 = 2k m, where m has at most two prime factors, and each of
them exceeds p1/10 . For such prime numbers p, we have
1
4
1 + 2k+1 β(m).
β(p − 1) = β(2k m) = β(2k )β(m) =
3
2
120
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
If m is prime, then
1
,
m(m − 1)
if m = r2 is a square of a prime, we have
β(m) = 1 +
β(m) = 1 +
r3 + 1
,
r3 (r2 − 1)
while if m = rs is a product of two distinct primes, then
1
1
β(m) = 1 +
1+
,
r(r − 1)
s(s − 1)
by Lemma 1. At any rate, with k fixed and p tending to infinity
through
4
1
prime numbers of the above form, we get that the number
1 + 2k+1
3
2
is a cluster point for the set B = {β(p − 1) : p prime}. Since this is true for
all positive integers k, we get that 4/3 is also a cluster point for B, which
takes care of (i).
For (ii), Theorem 1 says that the limit in (ii) is at most A. To show
equality, we let x be a large positive real number, write
Y
p,
Px =
p≤x
and let qx be the smallest prime number in the arithmetic progression
Px +1 mod Px2 , which exists by Dirichlet’s Theorem, since Px +1 is coprime
to Px2 . We have qx − 1 ≡ Px mod Px2 and may write
qx − 1 = Px mx ,
where each prime factor of mx is larger than x. Thus
Y
1
β(qx − 1) = β(Px )β(mx ) =
1+
· β(mx ),
p(p − 1)
p≤x
(4.6)
−1
Y
β(qx − 1)
1
1+
.
= β(mx ) ·
A
p(p − 1)
p>x
We now consider the prime factorization
mx = pe11 · · · pekk
of mx , where p1 , . . . , pk > x are distinct primes and e1 , . . . , ek are positive
integers. Then
Y
Y
1
1 < β(mx ) ≤
β(pi ) =
1+
pi (pi − 1)
1≤i≤k
1≤i≤k
(4.7)
X
Y
1
1
1+
<
< exp
= 1 + O(x−1 ).
p(p
−
1)
p(p
−
1)
x 2p − 1 ≥ p1 · · · pk > pk ,
so that k < p/ log2 p. Thus
k
Y
2p − 1
1
=
1
+
ϕ(2p − 1)
pi − 1
i=1
k
1
1 k
< exp
≤ 1+
< exp
= 1 + o(1),
p
p
log2 p
1 < β(2p − 1) <
which proves (4.8).
Acknowledgements
We are grateful to Wilfrid Keller for help with large primes, to Helmut
Prodinger for pointing out a reference, and to Karl-Heinz Indlekofer for
useful discussions.
The first author thanks the John Knopfmacher Centre for Applicable
Analysis and Number Theory for arranging his visit there during which
most of this research was done.
References
T. M. Apostol (1976), Introduction to Analytic Number Theory. Springer-Verlag, New
York.
P. T. Bateman (1972) . The distribution of values of the Euler function. Acta Arithmetica 21, 329–345.
C. K. Caldwell & Y. Gallot (2000), Some results for n! ± 1 and 2 · 3 · 5 · · · p ± 1.
Preprint.
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J. R. Chen (1973), On the representation of a large even integer as a sum of a prime
and a product of at most two primes. Scientia Sinica 16, 157–176.
P. D. T. A. Elliott (1985), Arithmetic functions and integer products, volume 272 of
Grundlehren der Mathematischen Wissenschaften. Springer-Verlag, New York.
K. Ford (1999), The number of solutions of φ(x) = m. Annals of Mathematics 150,
1–29.
H. Halberstam & H.E. Richert (1974), Sieve Methods. Academic Press.
G. H. Hardy & E. M. Wright (1962), An introduction to the theory of numbers.
Clarendon Press, Oxford. 1st edition 1938.
K.-H. Indlekofer (1980), A mean-value theorem for multiplicative functions. Mathematische Zeitschrift 172, 255–271.
K.-H. Indlekofer (1981), Limiting distributions and mean-values of multiplicative
arithmetical functions. Journal f¨
ur die reine und angewandte Mathematik 328, 116–
127.
W. Keller (2000). Private communication.
D. G. Kendall & R. A. Rankin (1947), On the number of Abelian groups of a given
order. Quarterly Journal of Mathematics 18, 197–208.
J. Knopfmacher (1972), Arithmetical properties of finite rings and algebras, and analytic number theory. II. Journal f¨
ur die reine und angewandte Mathematik 254, 74–99.
J. Knopfmacher (1973), A prime divisor function. Proceedings of the American Mathematical Society 40, 373–377.
J. Knopfmacher & J. N. Ridley (1974), Prime-Independent Arithmetical Functions.
Annali di Matematica 101(4), 153–169.
W. LeVeque (1977), Fundamentals of Number Theory. Addison-Wesley.
H. L. Montgomery (1970), Primes in arithmetic progressions. Michigan Mathematical
Journal 17, 33–39.
H. L. Montgomery (1987), fluctuations in the mean of Euler’s phi function. Proceedings
of the Indian Academy of Sciences (Mathematical Sciences) 97(1-3), 239–245.
A. G. Postnikov (1988), Introduction to analytic number theory. Volume 68 of Translations of Mathematical Monographs. American Mathematical Society, Providence, RI.
H. Riesel & R. C. Vaughan (1983), On sums of primes. Arkiv for Matematik 21(1),
46–74.
I. E. Shparlinski (1990), Some arithmetic properties of recurrence sequences. Matematicheskie Zametki 47(6), 124–131. (in Russian); English translation in Mathematical
Notes 47, (1990), 612–617.
P. J. Stephens (1969), An Average Result for Artin’s Conjecture. Mathematika 16(31),
178–188.
A. Walfisz (1963), Weylsche Exponentialsummen in der neueren Zahlentheorie. Number 15 in Mathematische Forschungsberichte. VEB Deutscher Verlag der Wissenschaften,
Berlin.
Average order in cyclic groups
Joachim von zur Gathen
Fakult¨
at f¨
ur Elektrotechnik, Informatik und Mathematik
Universit¨
at Paderborn, 33095 Paderborn, Germany
E-mail : gathen@upb.de
URL: http://www-math.upb.de/~aggathen/
Arnold Knopfmacher
The John Knopfmacher Centre
for Applicable Analysis and Number Theory
University of the Witwatersrand
P.O. Wits 2050, South Africa
E-mail : arnoldk@cam.wits.ac.za
URL: http://www.wits.ac.za/science/number_theory/arnold.htm
Florian Luca
Instituto de Matem´
aticas
Universidad Nacional Aut´
onoma de M´
exico
C.P. 58180, Morelia, Michoac´
an, M´
exico
E-mail : fluca@matmor.unam.mx
Lutz G. Lucht
Institut f¨
ur Mathematik
TU Clausthal, Erzstraße 1
38678 Clausthal-Zellerfeld, Germany
E-mail : lucht@math.tu-clausthal.de
URL: http://www.math.tu-clausthal.de/personen/lucht.html
Igor E. Shparlinski
Department of Computing
Macquarie University
Sydney, NSW 2109, Australia
E-mail : igor@comp.mq.edu.au
URL: http://www.comp.mq.edu.au/~igor
123
de Bordeaux 16 (2004), 107–123
Average order in cyclic groups
par Joachim von zur GATHEN, Arnold KNOPFMACHER,
Florian LUCA, Lutz G. LUCHT et Igor E. SHPARLINSKI
´sume
´. Pour chaque entier naturel n, nous d´eterminons l’ordre
Re
moyen α(n) des ´el´ements du groupe cyclique d’ordre n. Nous montrons que plus de la moiti´e de la contribution `a α(n) provient des
ϕ(n) ´el´ements primitifs d’ordre n. Il est par cons´equent int´eressant
d’´etudier ´egalement la fonction β(n) = α(n)/ϕ(n). Nous d´eterminons le comportement moyen de α, β, 1/β et consid´erons aussi
ces fonctions dans le cas du groupe multiplicatif d’un corps fini.
Abstract. For each natural number n we determine the average
order α(n) of the elements in a cyclic group of order n. We show
that more than half of the contribution to α(n) comes from the
ϕ(n) primitive elements of order n. It is therefore of interest to
study also the function β(n) = α(n)/ϕ(n). We determine the
mean behavior of α, β, 1/β, and also consider these functions in
the multiplicative groups of finite fields.
Section 1. Introduction
For a positive integer n, we determine the average order α(n) of the
elements in the additive cyclic group Zn of order n. The major contribution
to α(n) is from the ϕ(n) primitive elements in Zn , each of order n. We show
that, in fact, the other elements never contribute more than the primitive
ones do.
More precisely, we consider the relative version β(n) = α(n)/ϕ(n). With
A=
315 ζ(3)
ζ(2)ζ(3)
=
≈ 1.94359 64368
ζ(6)
2π 4
we have for n ≥ 2:
1 = lim inf β(n) < β(n) < lim sup β(n) = A.
n−→∞
n−→∞
We also determine the mean behavior of α, β, and 1/β, and discuss the
average order of elements in the multiplicative groups of finite fields. The
lower bounds for β are different for even and for odd characteristic.
Manuscrit re¸cu le 19 avril 2002.
108
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
The original motivation for this research was the usage of groups in
cryptography. Here one looks for cyclic groups of large order (preferably a
prime number). If we take a finite field and pick a random element from
it, how large can we expect its order to be? Intuition says that one should
avoid fields whose multiplicative group order is largely made up from small
prime factors. The results of this paper put this intuition on a firm basis.
Section 2. The average order
For a ∈ Zn , we denote by ord(a) its order in the additive group Zn .
Then ord(a) divides n, and for each divisor d of n, there are exactly ϕ(d)
elements in Zn of order d. Thus the average order in Zn is
1X
1 X
ord(a) =
dϕ(d).
α(n) =
n
n
a∈Zn
d|n
The main contribution is the term with d = n, and we normalize by it:
α(n)
.
β(n) =
ϕ(n)
Since 1/n and ϕ(n) are multiplicative functions of n, so is their Dirichlet
convolution α(n) (see Apostol 1976, Theorem 2.14), and also β(n). We first
determine their values in the case of a prime power.
Lemma 2.1. Let p be a prime and k ≥ 1 an integer. Then
pk+1
1
1
1
k
k
α(p ) =
+
1 + 2k−1 .
, β(p ) = 1 + 2
p + 1 pk (p + 1)
p −1
p
In particular β(1) = 1 < β(pk+1 ) < β(pk ) ≤ β(p) = 1 + 1/(p2 − p).
Proof.
We have
X
1 X i
1
k
2i−1
i
α(p ) = k
(p − 1) · p
p ϕ(p ) = k 1 +
p
p
1≤i≤k
0≤i≤k
=
p2k+1
pk (p
+1
,
+ 1)
p2k+1 + 1
1
1
β(p ) = k
1 + 2k−1 .
= 1+ 2
p −1
p
p (p + 1)(p − 1)pk−1
k
Theorem 2.2. For an integer n ≥ 2, we have the following inequalities.
Y
Y
1
1
< β(n) ≤
1+
< A.
(i) 1 ≤
1+ 2
p −1
p(p − 1)
p|n
p|n
(ii) 1 = lim inf β(n) < β(n) < lim sup β(n) = A.
n−→∞
n−→∞
Average order in cyclic groups
Proof.
We have
Y
p prime
=
Y
p prime
1
1+
p(p − 1)
=
109
Y 1 − p−1 + p−2
1 − p−1
p prime
p−6
ζ(2)ζ(3)
1−
=
= A.
−2
−3
(1 − p )(1 − p )
ζ(6)
Claim (i) now follows from the multiplicativity of β and the lemma. For
(ii), we clearly have 1 < β(n) < A for all n ≥ 2. When n ranges through
1
the primes, then β(n) = 1 + n(n−1)
tends to 1, and when nk is the product
of the first k primes, then limk→∞ β(nk ) = A.
1.8
1.6
1.4
1.2
1
0
200
400
600
800
1000
Figure 2.1. Relative average order β(n) for n ≤ 1000.
Figure 2.1 shows the behavior of β(n) for n ≤ 1000. The visible bands
at 1 = β(1), 1.5 = β(2), 1.17 ≈ β(3), for example, are created by numbers
of the form n = kp with small k and p either prime or having only large
prime factors, namely k = 1, 2, 3 for the bands mentioned.
We have seen that α(n) is firmly wedged between ϕ(n) and A · ϕ(n).
Since lim inf n→∞ ϕ(n)/n = 0, we also have
lim inf α(n)/n = 0.
n→∞
Theorem 4.4 below shows that this lower limit is even obtained on subsequences corresponding to the multiplicative groups of finite fields.
110
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
Our upper limit A occurs in several other contexts. Kendall & Rankin
(1947), Section 3, consider the number of divisors of n that are divisible
by the squarefree part of n, and show that its asymptotic mean value is
A. Knopfmacher (1973) gives a more precise description of the mean value,
and Knopfmacher (1972), Theorem 3.1 (vi), presents a generalization. The
moments of this function are studied in Knopfmacher & Ridley (1974),
Theorem 4.4. LeVeque (1977), Problem 6.5, determines A as the sum given
in (3.7) below, and shows that the asymptotic mean of 1/ϕ is Ax−1 log x.
The constant A also appears in Bateman (1972), Montgomery (1970), and
Riesel & Vaughan (1983).
Throughout the paper, log x is the natural logarithm of x.
Section 3. The mean average order
In this section, we determine the mean of the averaging functions α and
β, and of γ = 1/β. A pleasant feature, due to double averaging, is that the
error terms become rather small. We denote the average of an arithmetic
function g by g¯—not to be confused with complex conjugation:
1 X
g(n)
g¯(x) =
x
1≤n≤x
for x ≥ 1. There is a well-developed theory with many general results
about the existence of means of arithmetic functions, see Elliott (1985);
Indlekofer (1980, 1981); Postnikov (1988). However, those general results
do not imply the specific statements of this work.
The average α
¯ is connected to the constant
3ζ(3)
ζ(3)
=
≈ 0.36538 14847.
Cα =
2ζ(2)
π2
Theorem 3.1. The mean α
¯ of α satisfies
α
¯ (x) = Cα x + O((log x)2/3 (loglog x)4/3 ) for x ≥ 3.
We have
1 X 1X
1 X 1 X
1 X
α(n) =
dϕ(d) =
ϕ(d).
α
¯ (x) =
x
x
n
x
k
Proof.
1≤n≤x
1≤n≤x
d|n
1≤k≤x
d≤x/k
Walfisz (1963), Chapter IV, proves via exponential sum estimates that
x
¯
−
ϕ(x)
≤ c (log x)2/3 (loglog x)4/3 for x ≥ 3,
2ζ(2)
with some constant c. Now from
Z ∞
X 1
dt
1
≤
=
3
3
k
2(x − 1)2
x−1 t
xx
1≤d≤x
112
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
By inserting the estimates
Z ∞
X 1
dt
1
≤
=
,
2
2
d
⌊x⌋
⌊x⌋ t
d>x
Z x
X 1
dt
≤1+
= 1 + log x,
d
1 t
1≤d≤x
we see that for x ≥ 2
1
|R(x)| ≤ 1 + log x +
2
1
x
+
⌊x⌋ x
< 2 + log x.
This shows (i), and (ii) follows by inserting (i) into the proof of Theorem 3.1.
For two arithmetic functions f, g : N −→ C, f ∗ g is their Dirichlet convolution, with
X
(f ∗ g)(n) =
f (d)g(n/d)
d|n
for all n. Furthermore, we denote by 1 the constant function on N with
value 1, and µ is the M¨obius function.
Lemma 3.3. Let f and g be arithmetic functions with f = 1 ∗ g, and
consider the Dirichlet series
X g(n)
ge(s) =
.
ns
n≥1
(i) If ge(s) is absolutely convergent for ℜs ≥ 0, then the mean of f is
1
¯
,
f (x) = ge(1) + O
x
and more precisely
|f¯(x) − ge(1)| ≤
1X
|g(n)|
x
n≥1
for x ≥ 1.
(ii) If f is multiplicative and ge(s) converges absolutely for some s with
ℜs ≥ 0, then ge(s) can be written as the Euler product
Y
g(p) g(p2 )
ge(s) =
1 + s + 2s + · · · .
p
p
p prime
The absolute convergence of ge(s) is equivalent to
X |f (pk ) − f (pk−1 )|
< ∞.
pks
p prime
k≥1
Average order in cyclic groups
Proof.
113
For x ≥ 1, we have
X
X
x
g(d)
=
f
(n)
−
xe
g
(1)
−
xe
g
(1)
d
d≤x
n≤x
X g(d)
X
≤ x
− xe
g (1) +
|g(d)|
d
d≤x
≤x
d≤x
X |g(d)|
d>x
d
+
X
|g(d)| ≤
d≤x
X
|g(d)|,
1≤d
which implies (i).
If f is multiplicative, then so is g = µ ∗ f , and g(pk ) = f (pk ) − f (pk−1 )
for all primes p and k ∈ N . Now the Euler product representation of
ge(s) follows from the unique factorization
If ge(s) is absolutely
P theorem.
convergent, then so is the partial series
g(pk ) p−ks taken over all prime
powers pk . Conversely, absolute convergence of the latter series implies that
for any x ≥ 1
X |g(pk )|
Y
X |g(n)|
1+
≤
nℜs
pkℜs
p≤x
p prime
n≤x
≤
Y
p≤x
p prime
k≥1
exp
X
k≥1
|g(pk )|
pkℜs
≤ exp
X
p prime
k≥1
|g(pk )|
pkℜs
Thus ge(s) converges absolutely, which finishes the proof of (ii).
< ∞.
The mean of β is connected to the constant
105 ζ(3)
ζ(3)ζ(4)
=
≈ 1.29573 09579.
Cβ =
ζ(8)
π4
Theorem 3.4. The average value β¯ of β equals Cβ + O(x−1 ), and more
precisely
Y
p+2
−1
¯
|β(x) − Cβ | < x
1+ 3
p −p
p prime
for x ≥ 1.
Proof.
We use Lemma 3.3 with f = β and g = µ ∗ β. Thus
1
for k = 1,
1)
g(pk ) = β(pk ) − β(pk−1 ) = p(p −
1
−
for k ≥ 2,
p2k−1
114
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
for a prime p and an integer k ≥ 1 . Due to
X 1
X
p+2
1
= 3
+
,
(3.5)
|g(pk )| =
p (p − 1)
p −p
p2k−1
k≥2
k≥1
P
the series
|g(pk )| p−kℜs taken over all prime powers pk converges for
ℜs ≥ 0, and Lemma 3.3 (ii) implies the absolute convergence of the Dirichlet
series ge(s). In particular, we obtain
Y
X 1
Y
p+1
1
=
1
+
−
ge(1) =
1+ 2
p (p − 1)
p (p3 − 1)
p3k−1
p prime
=
Y
p prime
p prime
k≥2
Y
p−4
1+
=
1 − p−3
p−8
1−
ζ(3) ζ(4)
=
= Cβ .
(1 − p−3 )(1 − p−4 )
ζ(8)
p prime
Finally, Lemma 3.3 combined with (3.5) yields
X
Y
X
¯
|β(x)
− Cβ | < x−1
|g(d)| = x−1
1+
|g(pk )|
p prime
d≥1
= x−1
Y
1+
p prime
p+2
p3 − p
k≥1
≈ 2.26507 69892 · x−1 ,
which completes the proof.
¯
It is interesting to compare the behavior of β(x)
≈ ζ(3)ζ(4)/ζ(8) with
its naive “prediction” α(x)/
¯
ϕ(x)
¯
≈ ζ(3), see Theorem 3.1 and Lemma 3.2.
We have ζ(4)/ζ(8) ≈ 1.07792 81367.
Figure 3.1 shows the behavior of
Y
p + 2 −1
¯
1+ 3
(3.6)
(β(x)
− Cβ ) · x ·
p −p
p prime
for integer x ≤ 1000. Theorem 3.4 says that this quantity is absolutely
smaller than 1.
We can also express our constants A and Cβ as sums of Dirichlet series
via the Euler product decomposition
Y
X
f (n) =
(1 + f (p) + f (p2 ) + · · · ),
1≤n
p prime
which is valid for a multiplicative function f in the case of absolute convergence. Now
1+
X (µ(pk ))2
1
,
=
p(p − 1)
pk ϕ(pk )
0≤k
1+
X
1
(µ(pk ))2
=
p(p3 − 1)
pk ϕ(pk )σ(p2k )
0≤k
Average order in cyclic groups
115
0.05
200
400
600
800
1000
0
–0.05
–0.1
–0.15
–0.2
Figure 3.1. The average of β normalized as in (3.6).
imply that
(3.7)
A=
X (µ(n))2
1≤n
nϕ(n)
,
Cβ =
X
1≤n
(µ(n))2
.
nϕ(n)σ(n2 )
Both series seem to converge much slower than the product representations.
The mean of the function γ = 1/β = ϕ/α is connected to the constant
Y
1 X
1
1
1+ 1− 2
Cγ =
1−
≈ 0.80146 96934.
p
p
pk + p−k−1
p prime
1≤k
Theorem 3.8. The mean γ¯ of γ satisfies γ¯ (x) = Cγ + O(x−1 ), and more
precisely,
|¯
γ (x) − Cγ | ≤ Dx−1
for a constant D which is explicitly given in the proof below.
Proof. Again, we use Lemma 3.3, with the multiplicative function f = γ.
For a prime p and k ≥ 1, we have
f (pk ) =
1 − p−2
1
=
,
β(pk )
1 + p−2k−1
116
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
by Lemma 2.1. For the multiplicative function g = µ ∗ f we find
1 + p−1
− 2
if k = 1,
p (1 + p−3 )
g(pk ) = f (pk ) − f (pk−1 ) =
1
if k ≥ 2.
2k−1
−2k+1
p
(1 + p
)(1 + p−2k−1 )
Thus the Dirichlet series ge(s) is absolutely convergent for ℜs ≥ 0. We have
X g(n)
Y
X g(pk )
.
ge(1) =
=
1+
n
pk
1≤n
p prime
1≤k
For a prime p, the factor in this product equals
X f (pk ) − f (pk−1 )
X g(pk )
1 X
1 f (pk )
1
−
+
1+
=
1
+
=
1
−
p
p
pk
pk
pk
1≤k
1≤k
1≤k
1
1
1 X
= 1−
.
1+ 1− 2
p
p
pk + p−k−1
1≤k
Lemma 3.3 now implies that
|¯
γ (x) − Cγ | ≤
1X
|g(n)|,
x
1≤n
and the claim follows from the absolute convergence of ge(0). A numerical
evaluation of the error term gives
Y
X
X
k
|g(n)| =
1+
|g(p )| ≈ 1.96531.
D=
1≤n
p prime
1≤k
We have Cβ · Cγ ≈ 1.03848 90929.
Section 4. Finite fields
Our original motivation for this work was to study the average order
in the (cyclic) multiplicative group F×
q = Fq \{0} of a finite field Fq . We
first show that for the two families q = 2k and q a prime, α(q − 1)/(q − 1)
is on average between two positive constants, and also exhibit subfamilies
for which this quotient tends to zero. We also obtain several results for
β(q − 1).
Theorem 4.1. There are two absolute constants A2 ≥ A1 > 0 such that
for all K ≥ 1
1 X α(2k − 1)
A1 ≤
≤ A2 .
K
2k − 1
1≤k≤K
Average order in cyclic groups
Proof.
(4.2)
117
We use the asymptotic formula from Shparlinski (1990)
1 X ϕ(2k − 1)
= η + O(K −1 log K),
K
2k − 1
1≤k≤K
with η given by the absolutely convergent series
η=
X µ(d)
≈ 0.73192,
dtd
d odd
where td is the multiplicative order of 2 modulo d. The claim follows from
(4.2) and Theorem 2.2.
The proof of Theorem 4.1 implies that for any constant c < η, α(2k −1) ≥
c · (2k − 1) for infinitely many integers k. We may, of course, take A2 = 1
in Theorem 4.1; it is not clear whether Theorem 4.1 holds with a smaller
value of A2 . We also see that for any ε > 0 and sufficiently large values of
K, Theorem 4.1 holds with A1 = η − ε.
Stephens (1969) shows in his Lemma 1 that
X ϕ(p − 1)
p≤x
where
κ=
p
= κ lix + O(x/(log x)D ),
Y
X µ(d)
1
=
1−
≈ 0.37397
dϕ(d)
p(p
−
1)
p
d≥1
is Artin’s constant and D > 1 is arbitrary. The sum does not change by
much if we replace p by p − 1 in the denominator, since
X ϕ(p − 1)
p≤x
p−1
=
X ϕ(p − 1)
p≤x
p
+ O(log log x).
Using the bounds of Theorem 2.2 on β = α/ϕ, the fact that p − 1 is even
for p ≥ 3, and β(2) = 3/2, we find that
(4.3)
3κ
1 X α(p − 1)
−ǫ≤
≤ Aκ + ǫ,
2
x
p−1
p≤x
for any ǫ > 0 and sufficiently large x.
Thus there is an infinite sequence of fields of characteristic 2, and also one
of prime fields, in which the average order is close to its largest possible
value. Now we show that α(2k − 1) and α(p − 1) infinitely often take
relatively small values, just as ϕ(2k − 1) and ϕ(p − 1) do.
118
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
Theorem 4.4. For infinitely many integers k ≥ 3 and for infinitely many
primes p, we have
1
1
α(p − 1)
α(2k − 1)
=O
=O
and
.
loglog k
p−1
loglog p
2k − 1
Proof.
Let pi denote the ith prime. For an integer r ≥ 1, we put
kr = (p2 − 1) · · · (pr − 1)
and
mr = p2 · · · pr = nr /2.
Then mr divides 2kr − 1, and therefore
A · (2kr − 1)ϕ(m )
r
.
α 2kr − 1 < A · ϕ 2kr − 1 ≤
mr
Using the bound
mr
ϕ(mr ) = O
log(log mr + 1)
(see Hardy & Wright (1962), Theorem 328) and kr < mr , we obtain the
first statement.
To prove the second bound, we select qr as the smallest prime number
in the arithmetic progression 1 mod mr . Then
qr
A · (qr − 1)ϕ(mr )
=O
.
α(qr − 1) < A · ϕ(qr − 1) ≤
mr
log(log mr + 1)
From Linnik’s Theorem on the smallest prime number in an arithmetic
progression, we have log qr = O(log mr ), and the result follows.
In particular,
lim inf α(2k − 1)/(2k − 1) = lim inf α(q − 1)/(q − 1) = 0.
q prime
k→∞
Open Question. Obtain analogs of (4.2) and (4.3) for the sums
X
X
X α(p − 1)
X α(2k − 1)
k
β(p−1).
,
,
β(2
−1),
p−1
2k − 1
1≤k≤K
1≤k≤K
p≤K
p≤K
In the above we considered only α(2k − 1). Similar results also hold for
α(pk − 1) for any fixed p and growing k.
The convergence to zero of α(qr − 1)/(qr − 1) as above seems rather slow.
For the largest known “primorial prime” q = n33 237 + 1, where as before
nk is the product of the first k primes (see Caldwell & Gallot 2000), with
169 966 digits and the largest prime factor p33 237 = 392 113 of q − 1, we
have α(q − 1)/(q − 1) ≈ 0.0847. Also, β(q − 1) ≈ 1.94359 608 is close to A.
Concerning lower bounds for β, the situation is quite different between
characteristic 2 and odd characteristic.
Average order in cyclic groups
119
In a finite field F2k of characteristic 2, the group of units is cyclic with
2k − 1 elements. For a Mersenne prime Mk = 2k − 1, we have β(Mk ) =
1 + (Mk2 − Mk )−1 . If there are infinitely many of them, then lim inf β(2k −
1) = 1. For the current world record k = 69 72593 (see Chris Caldwell’s
web site http://www.utm.edu/research/primes), we have β(Mk ) ≈ 1 +
0.52 · 10−41 97919 .
For a field Fq of odd characteristic, 2 divides q − 1 = #F×
q and thus
β(q − 1) > 4/3, by Lemma 2.1. For a prime q = m2k + 1 with m odd, we
have
1
4
1 + 2k+1 · β(m).
β(q − 1) =
3
2
As an example, with the prime m = 10500 + 961 and k = 3103, q is indeed
prime (Keller 2000), and
4
β(q − 1) ≈ (1 + 10−1000 ).
3
We now prove the limits indicated by these experimental results.
Theorem 4.5. We have
(i) lim inf β(p − 1) = 4/3,
p prime
(ii) lim sup β(p − 1) = A,
p prime
(iii) lim inf β(2k − 1) = 1.
k→∞
Proof. To show that the limit in (i) is at least 4/3, we notice that if
p ≥ 3, then p − 1 = 2k m with some k ≥ 1 and some odd integer m, and
therefore
4
4
1
β(p − 1) = β(2k m) =
1 + 2k+1 β(m) > .
3
3
2
For the equality in (i), we use a theorem of Chen (see Chen (1973), or
Lemma 1.2 in Ford (1999), or Chapter 11 of Halberstam & Richert (1974))
which says that for each even natural number n there exists x0 such that
for every x ≥ x0 there exists a prime number p ∈ (x/2, x] with p ≡ 1 mod n
such that (p−1)/n has at most two prime factors, and each of them exceeds
x1/10 .
We now choose a positive integer k and apply Chen’s Theorem with
n = 2k to conclude that there exist infinitely many prime numbers p such
that p − 1 = 2k m, where m has at most two prime factors, and each of
them exceeds p1/10 . For such prime numbers p, we have
1
4
1 + 2k+1 β(m).
β(p − 1) = β(2k m) = β(2k )β(m) =
3
2
120
J. von zur Gathen, A. Knopfmacher, F. Luca, L. Lucht, I. Shparlinski
If m is prime, then
1
,
m(m − 1)
if m = r2 is a square of a prime, we have
β(m) = 1 +
β(m) = 1 +
r3 + 1
,
r3 (r2 − 1)
while if m = rs is a product of two distinct primes, then
1
1
β(m) = 1 +
1+
,
r(r − 1)
s(s − 1)
by Lemma 1. At any rate, with k fixed and p tending to infinity
through
4
1
prime numbers of the above form, we get that the number
1 + 2k+1
3
2
is a cluster point for the set B = {β(p − 1) : p prime}. Since this is true for
all positive integers k, we get that 4/3 is also a cluster point for B, which
takes care of (i).
For (ii), Theorem 1 says that the limit in (ii) is at most A. To show
equality, we let x be a large positive real number, write
Y
p,
Px =
p≤x
and let qx be the smallest prime number in the arithmetic progression
Px +1 mod Px2 , which exists by Dirichlet’s Theorem, since Px +1 is coprime
to Px2 . We have qx − 1 ≡ Px mod Px2 and may write
qx − 1 = Px mx ,
where each prime factor of mx is larger than x. Thus
Y
1
β(qx − 1) = β(Px )β(mx ) =
1+
· β(mx ),
p(p − 1)
p≤x
(4.6)
−1
Y
β(qx − 1)
1
1+
.
= β(mx ) ·
A
p(p − 1)
p>x
We now consider the prime factorization
mx = pe11 · · · pekk
of mx , where p1 , . . . , pk > x are distinct primes and e1 , . . . , ek are positive
integers. Then
Y
Y
1
1 < β(mx ) ≤
β(pi ) =
1+
pi (pi − 1)
1≤i≤k
1≤i≤k
(4.7)
X
Y
1
1
1+
<
< exp
= 1 + O(x−1 ).
p(p
−
1)
p(p
−
1)
x 2p − 1 ≥ p1 · · · pk > pk ,
so that k < p/ log2 p. Thus
k
Y
2p − 1
1
=
1
+
ϕ(2p − 1)
pi − 1
i=1
k
1
1 k
< exp
≤ 1+
< exp
= 1 + o(1),
p
p
log2 p
1 < β(2p − 1) <
which proves (4.8).
Acknowledgements
We are grateful to Wilfrid Keller for help with large primes, to Helmut
Prodinger for pointing out a reference, and to Karl-Heinz Indlekofer for
useful discussions.
The first author thanks the John Knopfmacher Centre for Applicable
Analysis and Number Theory for arranging his visit there during which
most of this research was done.
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Average order in cyclic groups
Joachim von zur Gathen
Fakult¨
at f¨
ur Elektrotechnik, Informatik und Mathematik
Universit¨
at Paderborn, 33095 Paderborn, Germany
E-mail : gathen@upb.de
URL: http://www-math.upb.de/~aggathen/
Arnold Knopfmacher
The John Knopfmacher Centre
for Applicable Analysis and Number Theory
University of the Witwatersrand
P.O. Wits 2050, South Africa
E-mail : arnoldk@cam.wits.ac.za
URL: http://www.wits.ac.za/science/number_theory/arnold.htm
Florian Luca
Instituto de Matem´
aticas
Universidad Nacional Aut´
onoma de M´
exico
C.P. 58180, Morelia, Michoac´
an, M´
exico
E-mail : fluca@matmor.unam.mx
Lutz G. Lucht
Institut f¨
ur Mathematik
TU Clausthal, Erzstraße 1
38678 Clausthal-Zellerfeld, Germany
E-mail : lucht@math.tu-clausthal.de
URL: http://www.math.tu-clausthal.de/personen/lucht.html
Igor E. Shparlinski
Department of Computing
Macquarie University
Sydney, NSW 2109, Australia
E-mail : igor@comp.mq.edu.au
URL: http://www.comp.mq.edu.au/~igor
123